In a cased well equipped with a wellhead, tubing strings are supported by a tubing hanger which is in turn supported by a tubing head in a manner well known in the art. Tubing heads are generally mounted to a surface flange of the cased well. The tubing hanger is received in the tubing head and locked in position by lock-down screws to ensure that the tubing hanger is not ejected from the tubing head if the well is subjected to significant fluid pressure. The tubing string is generally suspended by threaded attachment to the tubing hanger. The position of a bottom end of the tubing string is therefore fixed and determined by the length of the string. In order to change a position of the bottom end of the tubing string, a complicated process must be undertaken which involves removal of the wellhead and the tubing hanger. Before the wellhead can be removed, it is usually necessary to "kill" the well by overburdening any natural pressure to ensure that hydrocarbons do not escape from the well when the wellhead is removed. Operations such as killing a well and removing a wellhead require considerable time and generally involve the use of a derrick or a rig in order to handle components and ensure safety. Such operations therefore require the engagement of complex equipment and skilled labour which involves considerable expense.
It is therefore desirable to provide a method and apparatus to permit the axial displacement of a tubing string in a cased well bore without removal of the wellhead or necessity for killing the well. One such apparatus is described in applicant's copending patent application Ser. No. 08/946,510 entitled TELESCOPING JOINT FOR USE IN A CONDUIT CONNECTED TO A WELLHEAD AND ZONE ISOLATING TOOL FOR USE THEREWITH which was filed on Oct. 7, 1997. The disclosure of that application is incorporated herein by reference in its entirety. An apparatus for use in moving tubing connected to the telescoping joint was described in applicant's copending application Ser. No. 08/992,235 entitled APPARATUS FOR AXIALLY DISPLACING A DOWNHOLE TOOL OR A TUBING STRING IN A WELL BORE which was filed on Dec. 17, 1997. The disclosure of that application is likewise incorporated herein by reference in its entirety.
Although the telescoping joint described above greatly facilitates certain downhole operations, the axial displacement of a tubing string which may be achieved using the telescoping joint is limited by a length of the telescoping joint(s) in the tubing string. While that limited range of axial displacement is adequate for most downhole operations that require displacement of a bottom end of the tubing string, it is sometimes desirable to be able to displace the bottom end of the tubing string over a greater distance than is economically afforded by a telescoping joint(s).
There therefore exists a need for an apparatus and method which permits axial displacement of a tubing string in a cased well bore over a range which is practically limited only by the length of the tubing string itself.